Ultrasonic imaging technology has become a vital tool for examining the internal structure of living organisms. For the diagnosis of various medical conditions, ultrasonic imaging is often useful to examine soft tissues within the body to show the structural detail of internal tissues and fluid flow.
To examine internal body structures, ultrasonic images are formed by producing very short pulses of ultrasound using a transducer, sending the pulses through the body, and measuring the properties (e.g., amplitude and phase) of the echoes from tissues within the body. Focused ultrasound pulses, referred to as "ultrasound beams", are targeted to specific tissue regions of interest in the body. Typically, an ultrasound beam is focused at various steps within the body to improve resolution or image quality. Echoes are received by the transducer and processed to generate an image of the tissue or object in a region of interest. The resulting image is usually referred to as a B-scan image.
Measuring and imaging blood (and other bodily fluid) flow within a living subject is typically done using the Doppler principle, in which a transmitted burst of ultrasound at a specific frequency is reflected from moving blood cells, thereby changing the frequency of the reflected ultrasound in accordance with the velocity in the direction of the flow. The frequency shift (Doppler shift) of reflected signals with respect to the transmitted signals is proportional to the velocity of the fluid flow. This frequency may be detected and displayed on a video display device to provide graphic images of moving tissue structure and fluid flow within a living patient.
Present ultrasound techniques include frequency-shift color Doppler and power color Doppler imaging of tissue motion, as well as cross-correlation ultrasound estimation of displacements and mean velocities for color mapping tissue motion (referred to as CVI and developed by Philips Corp.). These present known methods of ultrasound imaging provide relatively limited information regarding the velocity and direction of flow in complex media. For example, present color Doppler flow imaging techniques (CDI) provide only frequency-shift data that is dependent on both the velocity of fluid flow or tissue motion and the Doppler angle between the ultrasound beam and the direction of flow or motion. This method provides neither the absolute velocity of the flow or motion, nor the direction of flow or motion.
The cross-correlation technique (CVI) produces and displays a limited range of velocities of flow or motion. Although CVI systems can be calibrated to produce absolute velocities and flow direction, the algorithms involved are complex and computationally intensive, thus requiring increased processing time and computer resources. Furthermore, because the uncertainty in cross-correlation estimates of velocity and direction tend to be large, the signal-to-noise ratio of this method is likely to be poor.